Digital data transmission provides communication in a variety of applications including, for example, communication over wired and wireless telephone system infrastructure and various types of area-defined networks. Cellular telephone communication systems are examples of existing telephone system infrastructure, involving both wired and wireless signal communication. Examples of area-defined networks include mobile wireless and digital subscriber link communication systems, and wireless personal area networks (“WPAN”). Each of these types of communication systems have associated standards that define the manner of communication between users over the wired and/or wireless communication channels.
The communication standards for these communication systems may define a frequency range or band that is shared by multiple users to permit simultaneous transmission of the users' respectively-defined information. For example, in multi-carrier communication using existing telephone lines, such systems transmit data using discrete frequency bands or subchannels over telephone-lines typically arranged in a binder with a number of wire pairs in each binder. Each of the multiple users is permitted to transmit simultaneously over an assigned one of wire pairs. In a CDMA (code-division-multiple-access) system, such as a CDMA cellular telephone system, multiple users are permitted to transmit simultaneously in the same frequency, temporal and spatial dimension; the users' respectively-defined information is encoded before transmission, and decoded at the receiver, using a spectral-based signal coding protocol.
An OFDM (orthogonal-frequency-division-multiplex) communication system is typically a multi-carrier system that transmits the respective users' information simultaneously as data bits encoded to multiple sub-carriers. This approach is directed to optimizing use of the allocated frequency band and is applicable, for example, to ADSL, Hiperlan/2, DAB, etc. A set of orthogonal sub-carriers together forms an OFDM symbol. Various approaches for implementing an OFDM system have been considered including the approach described by the IEEE 802.11a OFDM system.
Ideally, each of these systems would be implemented with optimal signal quality at the highest data transmission rate (or throughput). In a typical system, however, an increase in the data transmission rate compromises signal quality due to noise resulting from various system-related issues. For example, in a multi-carrier twisted-pair telephone-line system in which the twisted-pairs are bundled, crosstalk interference arises between twisted pairs arising from electromagnetic coupling within the binder that may degrade the communication signals. As the speed and/or power of data transmission increases, the crosstalk interference becomes more severe. CDMA-based and OFDM-based systems transmit the data from multiple users as symbols via the same, or a shared, frequency band, and a consequential noise concern includes inter-symbol interference (ISI). Thus, in each of these communication systems, signals from different users interfere with one another.
Various approaches have been investigated to address the adverse effects of such multi-user interference. In CDMA-based systems, a combination of user specific signature sequences, multi-user processing at the receiver and spatial processing is used to separate the signals from the different users. Ideally, the objective of the rate allocation is to maximize the achievable rate of each user given the transmit power constraints. However, because user signals interfere with one another, a change in the data rate of one user causes a change in power allocation for all users, and hence it is not possible to simultaneously maximize the rate of every user. Consequently, realizing an ideal system, one that properly allocates transmission rates to each of the users while attempting to maximize the system throughput, has been challenging.